Requirement of Insulin Growth Factor I plus Hydrocortisone for the Regeneration of Thyrotropin (TSH)-Dependent Mechanism of I- Efflux and Ca2+ Mobilization in FRTL-5 Cells during TSH Depletion

FRTL-5 rat thyroid cells grown in culture medium supplemented with serum and 6H (TSH, insulin, hydrocortisone, transferrin, glycylhistidyllysine, and somatostatin) showed a significant increase in TSH-dependent cAMP accumulation and I- efflux after prolonged incubation (5 to 10 days) of the cells in culture medium containing 5H (6H – TSH) or serum. The induction of the cAMP response was at least partly reproduced when both serum and 5H were omitted from the medium. However the I- efflux response was completely abolished under such conditions and regenerated when serum or 5H was present. The serum or 5H effect on I- efflux response was mimicked by 2H (insulin + hydrocortisone). Insulin was replaced by 1/1000 less insulin-like growth factor-I than insulin. TSHdependent Ca2+ mobilization of the cells was similarly affected by the presence of serum or 2H. However, the I- efflux and Ca2+ responses to an agonist other than TSH (extracellular ATP) were not substantially influenced by serum and/or 2H as well as T...